Iron oxide magnesium oxide composites and method for destruction of cholrinated hydrocarbon using such composites

ABSTRACT

Finely divided composite materials are provided comprising a first metal oxide (e.g., MgO) at least partially coated with an extremely thin layer of a second metal oxide such as Fe 2  O 3 . The composites have a high surface area and are very effective for the destructive adsorption of undesirable fluids in gaseous or liquid form, such as chlorocarbons and chlorofluorocarbons. In use, a fluid stream including undesirable fluids are contacted with the composites of the invention, such as through the use of a filter containing the composite as a part of the filter media thereof.

This invention was made with government support under GrantDAAH04-93-G-0328 awarded by the United States Army Research Office. Thegovernment has certain rights in the invention.

This application is a continuation of application Ser. No. 08/224,705,filed Apr. 8, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with finely divided(preferably nanoscale) composite materials which have significantutility in the destruction of chlorocarbons, fluorocarbons and othertoxic or undesirable fluids. More particularly, the invention pertainsto composite materials including first metal oxide particles, preferablyMgO, at least partially coated with an extremely thin layer of a secondmetal oxide such as Fe₂ O₃. Composites in accordance with the inventionhave greatly enhanced abilities to destructively adsorb undesirablefluids such as gases or liquids.

2. Description of the Prior Art

The increasing amounts of chlorocarbons and chlorofluorocarbons in theenvironment has given rise to the need to find easy and effective waysto destroy these gases without producing toxic byproducts. The mostwidespread groups of chlorinated compounds are the polychlorinatedbiphenyls (PCBs) which have extremely high thermal stabilities and areused as lubricants, heat transfer fluids, plasticizers, and transformerfluids; and cleaning solvents such as CCl₄, CHCl₃, and C₂ Cl₄.Additionally, large stores of obsolete or overaged pesticides,herbicides, mixed wastes and nerve gases exist around the world, and thesafe and effective destruction of these materials is of increasingconcern.

These considerations have prompted a number of investigations todetermine the most feasible way of handling and destroying variousundesirable substances. Some of these destructive techniques involveincineration or catalytic oxidation. Another approach depends on surfaceactive reagents that strip heteroatoms from the toxic gases and allowonly the release of non-toxic hydrocarbons or carbon oxides. Forexample, the destructive adsorption of organophosphorus compounds on MgOcauses the phosphorus atoms to be immobilized as a strongly boundresidue, with the only volatile organic products being CH₃ OH and HCOOH.This same chemistry has been applied to the destruction of chlorocarbonsusing reactants such as MgO or CaO.

While destructive adsorption techniques appear promising and havefavorable thermodynamics, the cost thereof has been considerable, owingprincipally to the fact that, to be effective, the adsorptive reagentsmust be very finely divided for maximum surface area. Moreover, thesereactions are non-catalytic and depend entirely upon molecular reactionsat the surface of the reagents.

This is accordingly a real and unsatisfied need in the art for improveddestructive adsorption reagents which have enhanced destructiveefficiencies.

SUMMARY OF THE INVENTION

The present invention overcomes the problems outlined above and providesfinely divided composite materials useful for the destructive adsorptionof a wide variety of undesirable fluids (i.e., materials in eithergaseous or liquid form), such as chlorocarbons, chlorofluorocarbons,polychlorinated biphenyls, pesticides, herbicides, mixed wastes andnerve gases. Broadly speaking, the composites of the invention comprisefinely divided particles of a first metal oxide selected from the groupconsisting of MgO, CaO, Al₂ O₃, SnO₂, TiO₂, and mixtures thereof, theseparticles being at least partially coated with a quantity of a secondmetal oxide selected from the group consisting of Fe₂ O₃, Cu₂ O, NiO,CoO and mixtures thereof.

In preferred forms, the first metal oxide is advantageously selectedfrom the group consisting of MgO and CaO, whereas the second oxide ispreferably Fe₂ O₃. For most efficient usages, the particles of the firstmetal oxide should be single crystallites or polycrystalliteaggregations and should have an average size of up to about 20 nm, andmore preferably from about 4-10 nm; the second metal oxide should be inthe form of an extremely thin layer or coating applied onto the surfaceof the first metal oxide, giving an average overall size for thecomposite of up to about 21 nm, and more preferably from about 5-11 nm.The bulk composites of the invention should have an average surface areaof at least about 100 m² /g, and more preferably from about 300-500 m²/g.

Generally, the first metal oxide should be present in substantial excessrelative to the second oxide. Thus, the first metal oxide comprises fromabout 90-99% by weight of the total composite material, and morepreferably from about 95-99% by weight. Correspondingly, the secondmetal oxide should comprise from about 1-10% by weight of the totalcomposite, and more preferably from about 1-5% by weight. The coverageof the first oxide by the second oxide should be quite extensive, e.g.,at least about 75% of the surface area of the first metal oxideparticles should be covered with the second oxide, and more preferablyfrom about 90-100% of this surface area should be covered.

The composites of the invention are preferably fabricated by firstforming the very finely divided first particulate material using knownaerogel techniques. Thereafter, the second material is applied onto thesurface of the first oxide as an extremely thin layer, e.g., a monolayerhaving a thickness on the order of less than 1 nm. For example, MgOnanoscale particles can be prepared, and are then treated with an ironsalt such as iron III (acetylacetonate)₃ with the ligands being drivenoff by heating.

The composites of the invention can be used for the destruction ofunwanted fluids, especially gases in a gas stream, although theinvention is not limited to destruction of gases. The method may involvepassing a fluid stream through a filtering device including thecomposite material therein. In practice, the concentration of theunwanted fluids should be from about 0.3-1.3 g of the unwanted fluidsfor each gram of the composite material used for destructive adsorption.Generally, to be effective, the composites hereof should be heatedduring contact with the fluids to be destructively adsorbed. Thiselevated temperature should be at least about 300° C., and morepreferably from about 400°-600° C., in the case of materials such as thechlorocarbons, fluorocarbons and nerve gases. In order to destructivelyadsorb other materials such as the PCBs, higher temperatures may berequired on the order of 600°-800° C.

Although not wishing to be bound by any theory, it is believed that thesecond or outer oxide plays a catalytic role in the destructiveadsorption process, via a mechanism referred to as spillover catalysis.For example, when CCl₄ attacks the surface of an MgO/Fe₂ O₃ composite,it is believed that a disassociative adsorption occurs, probably withthe intermediacy of chemisorbed CCl₂. At this point, COCl₂ may form asan intermediate which goes on to react in a similar mode to produce CO₂and FeCl_(x). However, the important point is that the FeCl_(x) formedmust quickly exchange (spillover) Cl⁻ with O²⁻ in such a way that Cl⁻ isdriven into the bulk of the first oxide while O²⁻ comes to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the apparatus used in theproduction of Fe₂ O₃ /MgO composites of the invention;

FIG. 2 is a schematic representation of apparatus used in thechlorocarbon decomposition tests using the composites of the invention;

FIG. 3 is the infrared spectra of the gaseous products of CCl₄ reactedwith MgO (Graph a) and Fe₂ O₃ /MgO (Graph b), at a sample/CCl₄ molarratio of 3:1;

FIG. 4 is the infrared spectra of the gaseous products of CCl₄ reactedwith MgO (Graph a) and Fe₂ O₃ /MgO (Graph b), at a sample/CCl₄ molarratio of 6:1;

FIG. 5 is the infrared spectra of the gaseous products of CCl₄ reactedwith MgO (Graph a) and Fe₂ O₃ /MgO (Graph b), at a sample/CCl₄ molarratio of 10:1;

FIG. 6 is the infrared spectra of the gaseous products of CCl₄ reactedwith MgO (Graph a) and Fe₂ O₃ /MgO (Graph b), at a sample/CCl₄ molarratio of 13:1;

FIG. 7 is a gas chromatograph plot illustrating the percentagecomposition of gaseous products from the reaction between 0.1 g MgO andCCl₄ with respect to the microliters CCl₄ injected at 400° C. sampletemperature; and

FIG. 8 is a gas chromatograph plot illustrating the percentagecomposition of gaseous products from the reaction between 0.1 g Fe₂ O₃/MgO and CCl₄ with respect to the microliters CCl₄ injected at 400° C.sample temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate preferred embodiments of the inventionand use thereof. It is to be understood, however, that these examplesare presented by way of illustration only and nothing therein should betaken as a limitation upon the overall scope of the invention.

EXAMPLE 1

A. Preparation of MgO

Highly divided nanoscale MgO samples were prepared by the autoclavetreatment described by Utamapanya et al., Chem. Mater., 3:175-181(1991). In this procedure, 10% by weight magnesium methoxide in methanolsolution was prepared and 83% by weight toluene solvent was added. Thesolution was then hydrolyzed by addition of 0.75% by weight waterdropwise while the solution was stirred and covered with aluminum foilto avoid evaporation. To insure completion of the reaction, the mixturewas stirred overnight. This produced an aerogel which was treated in anautoclave in using a glass lined 600 ml capacity Parr miniature reactor.The gel solution was placed within the reactor and flushed for 10minutes with nitrogen gas, whereupon the reactor was closed andpressurized to 100 psi using the nitrogen gas. The reactor was thenheated up to 265° C. over a 4 hour period at a heating rate of 1°C./min. The temperature was then allowed to equilibrate at 265° C. for10 minutes.

At this point, the reactor was vented to release the pressure and ventthe solvent (final reactor pressure was about 700 psi). Finally, thereactor was flushed with nitrogen gas for 10 minutes. This produced finewhite powdery magnesium hydroxide having a surface area on the order of1000 m² /g which was then thermally converted to MgO as explained below.

B. Preparation of Fe₂ O₃ /MgO Composites

The Mg(OH)₂ particles were first thermally converted to MgO, followed bydeposition of iron oxide to provide the complete composite. Theapparatus employed for this operation is set forth in FIG. 1. Thisapparatus 10 includes a reactor 12 for holding a quantity of thepowdered magnesium hydroxide, with a shiftable magnetic stirrer 14positioned below the reactor and operable to rotate a stirrer bar 16.The apparatus further includes a mercury-filled 50 ml gas burette 18,manometers 20 and 21, a helium source 22 coupled with a molecular sieve13X trap 24, and a septa 26 operably coupled with reactor 12. A vacuumline 28 is also provided which is coupled via conduit and valve assembly30 to the remainder of the apparatus 10 as shown. Also, the reaction 12is equipped with an external, electrically controlled heater 31.

The initial thermal conversion of magnesium hydroxide to MgO was carriedout by heating the magnesium hydroxide in reactor 12 under dynamicvacuum conditions at an ascending temperature rate to a maximumtemperature of 500° C., which was held for 6 hours. Most of thedehydration was found to occur at temperatures between 200° C. and 320°C. IR and X-ray diffraction studies confirmed virtually completeconversion of the Mg(OH)₂ to MgO.

Iron oxide was deposited on the nanoscale MgO particles by carrying outa direct reaction between activated MgO and iron III (acetylacetonate)₃,in tetrahydrofuran at room temperature under helium (1 atm.).

In a typical preparation, 0.3 g of Mg(OH)₂ was heated under vacuum (10⁻³Torr.) in the reactor 12 at an ascending temperature rate of 1° C./min.to 500° C., which was held for 6 hours to assure complete conversion toMgO, followed by cooling to room temperature.

The evacuated system was then filled with He at 1 atm. pressure, andstirring was commenced using stirrer 14 and bar 16, the latter beingmagnetically shifted to the bottom of reactor 12. Two ml of 0.25M ironIII (acetylacetonate)₃ in THF solution (previously prepared under argonby dissolving 4.5 g of iron III (acetylacetonate)₃ in 50 ml THF) wasintroduced to the reactor 12 by a syringe through a septa 26. The amountof iron III (acetylacetonate)₃ solution used provided the MgO surfaceswith 1.4 iron III (acetylacetonate)₃ molecules for each surface OHgroup. The concentration of surface OH groups for the autoclave-preparedMgO was found to be 3.6 OH groups/nm². The reaction mixture in reactor12 was stirred overnight to allow a complete reaction at roomtemperature. The reacted Fe₂ O₃ /MgO composite was then removed from thereactor 12, filtered using regular filter paper, washed with THF toremove any residuals of iron III (acetylacetonate)₃, and dried in airfor 10 minutes.

IR spectra of the resultant dried product showed bands for theacetylacetonate species, indicating the existence of someacetylacetonate ligands bound to the surfaces of the MgO. This productwas heated again under vacuum (10⁻³ Torr.) at 500° C. to remove theseligands.

C. Chlorocarbon Decomposition Using MgO and the Fe₂ O₃ /MgO Composite

Two different studies were carried out to investigate and compare theefficiency of activated MgO and Fe₂ O₃ /MgO samples in decomposing CCl₄.Previous investigations using CaO for CCl₄ destruction indicated thatthe best decomposition occurs at 400° C. Therefore, in the following twosystems, the reaction temperature was 400° C.

In the first IR and XRD (X-Ray Diffraction) study, the apparatus of FIG.2 was employed. This apparatus 32 included two Schlenk tubes 34, 36respectively containing the MgO or Fe₂ O₃ /MgO composite and dry CCl₄, acalibrated 0.1 ml vial 38 and a vacuum source 40. A conduit and valveassembly 42 interconnected the source 40 with the tubes 34, 36 and vial38 as shown. Also, tube 34 and vial 38 were equipped with externalliquid nitrogen immersion containers 44, 46 for selective cooling of thecontents of the tube 34 and vial 38.

In test procedures, the MgO or Fe₂ O₃ /MgO was heated by means not shownin FIG. 2 under vacuum (10⁻³ Torr.) for complete activation (3 hours fora 0.2 g sample), with the CCl₄ vessel remaining closed via theappropriate valve. Next, 0.1 ml CCl₄ was collected in the calibratedvial 38 using external cooling via liquid nitrogen container 46, and byopening the valve adjacent the tube 36. After the CCl₄ was collected invial 38, and tube 34 had sufficiently cooled, the external liquidnitrogen container 44 was employed to further cool the contents of tube34, and the valve adjacent tube 34 was opened. This resulted in transferof the CCl₄ from vial 38 to tube 34 (the above steps were repeated whenmore than 0.1 ml of CCl₄ was needed). In practice, different molarratios of MgO or Fe₂ O₃ /MgO to CCl₄ were reacted for subsequenttesting.

The tube 34 containing the MgO or Fe₂ O₃ /MgO and the added CCl₄ wasclosed and heated at 400° C. for 9 hours. Next, the CCl₄ tube 36 wasreplaced by an IR gas cell with KBr windows, followed by evacuation(10⁻³ Torr.) of the system with the valve associated with tube 34 stillclosed. The tube 34 was then opened by opening the adjacent valve, inorder to transfer the gases produced from the reaction to the IR cell.After each such experiment, an IR spectrum was recorded for the gaseousproduct, and an XRD spectrum was recorded for the solid products.

IR spectra of the gaseous products showed bands for CO₂, C₂ Cl₄, COCl₂,CCl₄, and weak bands for HCl and CO gases (FIGS. 3-6). As indicated fromthe band intensities, the composition of these products changes as thesample/CCl₄ ratio changes. At low sample CCl₄ molar ratio 3:1), all ofthese gases are produced. As the ratio increases, the sample becomes inexcess and decomposes all of the CCl₄ and the intermediate products,COCl₂ and HCl. Comparing the spectra of the MgO and Fe_(O) ₃ /MgOreaction products, the following results are manifest.

1. The disappearance of COCl₂ (phosgene) in the case of Fe₂ O₃ /MgO wasfaster than in the case of MgO (FIG. 4) as the ratio was increased,which means that Fe₂ O₃ /MgO produces less phosgene at all ratios.

2. The production of C₂ Cl₄ was generally higher in the reaction of Fe₂O₃ /MgO.

3. The most striking feature was that the remaining (undecomposed) CCl₄was less in reaction of Fe₂ O₃ /MgO as shown by the less intense bandsof CCl₄ in the case of Fe₂ O₃ /MgO at all ratios, especially at ratiosof 6:1 and 10:1.

These confirm that Fe₂ O₃ composites are more efficient for destructionof CCl₄, in terms of decomposing more CCl₄, and producing less COCl₂gas.

The X-Ray diffraction spectra of the solid products showed patterns forMgCl₂, hydrated MgCl₂ and MgO. The hydration of some MgCl₂ was due tothe exposure of the samples to air during the XRD experiments. Comparingthe spectra of the reaction products of the Fe₂ O₃ /MgO compositesystems with the reaction products of the MgO systems, it was determinedthat:

1. At low molar ratios (3:1 and 6:1), MgO is a major component in thereaction products of the MgO systems, while it disappears almostcompletely in the reaction products of the Fe₂ /MgO systems.

2. Fe₂ O₃ /MgO systems produce more MgCl₂ than do the MgO systems, asshown by the stronger patterns of MgCl₂ in the spectra of the Fe₂ O₃/MgO systems.

3. Thus, more CCl₄ reacted with the Fe₂ O₃ /MgO composites, as comparedwith the same amount of MgO, thereby confirming the greater efficiencyof the Fe₂ O₃ /MgO composites.

In a second GC (gas chromatography) study, the gases produced from therapid reaction between a series of 1 μL injections (2 min. apart) ofCCl₄ through 0.1 g of MgO or Fe₂ O₃ /MgO composite at 400° C. For thistest, a 0.1 g sample of MgO or Fe₂ O₃ /MgO composite was packed in aU-shaped stainless steel reactor using glass wool and glass beads onother side or the sample to hold it in place. The reactor was connectedto the column of a GC instrument (TCD GC Series 580, GOW-MAC InstrumentCo.), and heated to 400° C. over 1 hour. The instrument was calibratedat a column and injector temperature of 100° C. He was used as a carriergas at a flow rate of 0.5 cm/s. CCl₄ was injected in 1 μL portions overthe heated sample and the results were recorded after each injection forthe gases coming out of the reaction tube.

The main gaseous products detected were CO₂, C₂ Cl₄ and unreacted CCl₄,as identified by a mass spectrometer connected to the GC instrument. Gaschromatography was used to study the gaseous products of a shortduration reaction between 0.1 g of MgO or Fe₂ O₃ /MgO and a series of 1μL portions of CCl₄ injected over the sample at 400° C. allowing 2minutes between injections. The main gaseous products detected were CO₂,C₂ Cl₄, and the unreacted CCl₄ as identified by a mass spectrometerconnected with a GC instrument in a separate system.

Generally, CO₂ gas was the major product along with traces of C₂ Cl₄ inthe first few CCl₄ injections. After a certain number of injections,CCl₄ became in excess and started to appear with the reaction products.The concentration of CCl₄ increased slowly with injections of more CCl₄until the sample was saturated and unable to decompose more CCl₄, asshown in FIGS. 7-8. As more CCl₄ was injected, the CO₂ and C₂ Cl₄production decreased but they continued to form after the saturation ofthe sample, which indicated that some decomposition continued to takeplace under these conditions. The results of two typical experimentsusing MgO and Fe₂ O₃ /MgO samples were illustrated in FIGS. 7 and 8,where the percent composition of the gaseous products is plotted withrespect to the amount of CCl₄ injected. The percentage of each gas wasmeasured by the percentage of its peak height after each injection.

The number of injections in which CCl₄ did not appear with the productsgives the amount of CCl₄ completely decomposed by a 0.1 g sample. Theefficiency of Fe₂ O₃ /MgO composites in decomposing CCl₄ was comparedwith that of MgO by carrying out the reaction with CCl₄ on the sameamount of both samples (0.1 g) under the same conditions. In all of theexperiments carried out, the difference in the behavior of both samplestoward CCl₄ was great. In the MgO reaction (FIG. 7), CCl₄ was completelydecomposed in the first 3 injections and the sample was saturated with15 μL of CCl₄. In contrast, the Fe₂ O₃ /MgO sample was saturated with 75μL of CCl₄ and 49 μL were completely decomposed. Depending on theseresults the sample/CCl₄ molar ratio for complete CCl₄ decomposition was80:1 and 5:1 for MgO and Fe₂ O₃ /MgO respectively. In other words, todecompose 1 mole of CCl₄, we need 80 moles of MgO or 5 moles of Fe₂ O₃/MgO. Another difference was that C₂ Cl₄ was not produced from the first10 injections in the Fe₂ O₃ /MgO system and CO₂ was the only product,while it was produced from the first injection in the MgO reaction. Thisconfirms that the Fe₂ O₃ /MgO composites are very much more efficient interms of decomposing CCl₄ and producing more CO₂, as compared with MgO.

D. Effect of Iron Content

The GC apparatus was also used to determine the effect of varying theiron content in the Fe₂ O₃ /MgO composites insofar as CCl₄ destructionis concerned. It was determined that the amount of iron deposited on theMgO surface increased the efficiency of the composites. Table 1 setsforth the amount of CCl₄ completely decomposed by Fe₂ O₃ /MgO compositeshaving differing amounts of iron, where the GC experiments were carriedout as set forth above.

                  TABLE 1                                                         ______________________________________                                        % Fe (by weight)                                                                          μL of CCl.sub.4 Completely Decomposed                          ______________________________________                                        0           3                                                                 1.7         21                                                                2.1         38                                                                2.2         49                                                                ______________________________________                                    

E. Effect of Other Reactants The preferred Fe₂ O₃ /MgO composites werecompared with other related samples by GC testing as described above.The other samples were: MgO mixed with iron III (acetylacetonate)₃ usingdifferent amounts of iron; MgO mixed with α-Fe₂ O₃ using differentamounts of iron; straight α-Fe₂ O₃ ; and straight iron III(acetylacetonate)₃. Table 2 below sets forth the amounts of CCl₄ (in μL)completely decomposed by 0.1 g of different samples at 400° C.

                  TABLE 2                                                         ______________________________________                                        Sample % Fe 0%    1.7%    2.1% 2.2%   2.5% 5.0%                               ______________________________________                                        Fe.sub.2 O.sub.3 /MgO                                                                     3     21      38   49     --   --                                 MgO/Fe(aca- 3     --      --   24     --   23                                 C).sub.3                                                                      MgO/α-Fe.sub.2 O.sub.3.sup.1                                                        3     --      --   --     5    --                                 α-Fe.sub.2 O.sub.3                                                                  11                                                                Fe (acac).sub.3                                                                           13                                                                ______________________________________                                         .sup.1 A physical mixture of separate MgO and Fe.sub.2 O.sub.3 particles.

F. The Effect of Time Between Injections

The time between CCl₄ 1 μL injections in the GC test described above wasfound to have a significant influence on the efficiency of the Fe₂ O₃/MgO composite samples in reacting with CCl₄. The results of this seriesof experiments, carried out using different time periods between CCl₄injections, but with all other conditions constant, is set forth inTable 3.

                  TABLE 3                                                         ______________________________________                                                   μL of CCl.sub.4 Completely Decomposed                           Time (minutes)                                                                             Sample A*   Sample B*                                            ______________________________________                                        2            24          30                                                   5            40          --                                                   10           --          60                                                   ______________________________________                                         *A and B are typical Fe.sub.2 O.sub.3 /MgO samples                       

The significant increase in the reactivity of Fe₂ O₃ /MgO toward CCl₄ byincreasing the time between CCl₄ injections indicates that longer timeshelp the sample to regenerate it surface structure and potential. Thismeans that Cl/O exchange process goes toward completion if longer timeis allowed. Based on these tests, the described spillover catalysismechanism is proposed. When short time intervals between CCl₄ injectionswere used, two things might happen to inhibit the Cl/O exchange: first,the Fe--Cl/Fe--O exchange may not occur for all of the surface Fe--Clbonds. Second, the exchange process would be expected to start withoxygen ions in the outer layers of MgO particles, which will be blockedwith Cl ions after a short time, covering the bulk oxygen ions. Whenlonger time intervals were used, these problems were somewhatalleviated: first, enough time was allowed for a complete Fe--Cl/Fe--Oexchange on the surface; second, the Cl ions accumulating in the outerlayers migrate over time, uncovering more O⁻² ions for the exchangeprocess.

EXAMPLE 2

In this example, a CaO/Fe₂ O₃ composite was prepared and tested for itsability to destructively adsorb chlorocarbons. The apparatus employedwas identical to that shown in FIG. 1. Commercially available Ca(OH)₂was first thermally activated to CaO under the dynamic vacuum andheating conditions recited in Example 1 to yield CaO having a surfacearea of about 100-120 m² /g. The CaO was then stirred for several hoursin a tetrahydrofuran solution of iron III (acetylacetonate)₃ under argonat the conditions of Example 1. The THF was then removed under vacuum,leaving an orange solid. This solid was heat treated again to 500° C. todrive off the acetylacetonate ligands. A gray/black powder resulted dueto the carbon present. The composite was then stored and handled in adry box to prevent reaction with moisture. X-ray diffraction studies ofthe sample detected from the atmosphere showed only the presence of CaO.

The CaO/Fe₂ O₃ composite was tested for destructive adsorption of CCl₄and found to be very efficient for this purpose.

We claim:
 1. A composite material comprising particles of a first metaloxide selected from the group consisting of MgO, CaO, Al₂ O₃, SnO₂, TiO₂and mixtures thereof, at least about 75% of the surface area of saidfirst metal oxide particles being coated with a quantity of a secondmetal oxide comprising Fe₂ O₃, said composite having an average particlesize of up to about 21 nm and a surface area of at least about 100 m²/g.
 2. The composite material of claim 1, said particles having anaverage size of up to about 20 nm.
 3. The composite material of claim 2,said average being from about 4-10 nm.
 4. The composite material ofclaim 1, said average surface area being from about 300-500 m² /g. 5.The composite material of claim 1, said first metal oxide comprisingfrom about 90-99% by weight of the total composite material.
 6. Thecomposite material of claim 5, said first metal oxide comprising fromabout 95-99% by weight of the total composite material.
 7. The compositematerial of claim 1, said second metal oxide comprising from about 1-10%by weight of the total composite material.
 8. The composite material ofclaim 7, said second metal oxide comprising from about 1-5% by weight ofthe total composite material.
 9. The composite material of claim 1, saidfirst metal oxide being MgO.
 10. The composite material of claim 1, saidsecond metal oxide being present as a layer having a thickness of up toabout 1 nm.
 11. The composite material of claim 1, said second metaloxide consisting essentially of Fe₂ O₃.
 12. A composite materialcomprising particles of a first metal oxide selected from the groupconsisting of MgO, CaO, Al₂ O₃, SnO₂, TiO₂ and mixtures thereof, saidparticles being at least partially coated with a quantity of a secondmetal oxide selected from the group consisting of Fe₂ O₃, Cu₂ O, CoO andmixtures thereof, said composite having an average particle size of upto about 21 nm and an average surface area of from about 300-500 m² /g.13. The composite material of claim 12, said particles having an averagesize of up to about 20 nm.
 14. The composite material of claim 12, saidfirst metal oxide comprising from about 90-99% by weight of the totalcomposite material, and said second metal oxide comprising from about1-10% by weight of the total composite material.